1. Field of the Invention
This invention relates to motorized drive device for cameras comprising an electric motor and a transmission system operating with selection of at least two speed change ratios.
2. Description of the Related Art
Recently, the automatic drive device by a motor has been widely used in cameras. For the charging of the shutter and the lens, and the winding-up and rewinding of film are driven by a single motor or a plurality of motors, a great improvement of the manageability has been achieved. In such a trend, it has been proposed that the windup transmission system and other like systems each have a plurality of speed reduction ratios, and these speed reduction ratios are selectively set automatically depending on the state of the electrical power source, the state of load, and other situations, so that the motor is driven under optimum condition, in the U.S. patent application Ser. Nos. 889,087, 865,063 and 855,799 assigned to the assignee of the present invention. The use of such a device or even another one in which a desired speed reduction ratio is selected by the photographer enables the mechanical stopping measures to be omitted when the motor is stopped by electrically detecting the completion of winding-up or the completion of the charging. But, the motor is rotating at a very high speed and cannot stop suddenly when the stop signal is given. So, it takes some time to stably stop an object to be driven as measured from the moment at which the stop signal for the motor has appeared. In that case, because different speed reduction ratios of the transmission system have different inertias of the transmission system including the motor, the time from the stop signal for the motor to the stop of the object to be driven and the distance the object overruns differ with different speed reduction ratios. In other words, for the speed reduction ratio to the high speed, the inertia is small and the time till the stop is short, but the overrun distance is long. For the case of the speed reduction ratio for the low speed, the inertia is small, and the stopping time is long, but the overrun distance is short. If the overrun distance is long, many drawbacks arise. For example, the charge system results in overcharging its load.
Also, as a countermeasure against the difference in the stopping time or stabilization time based on the above-described fact of the difference of speed reduction ratio, a room for surplus of the stabilization time must be created, and, upon having waited for the elapse of this longer time than whichever stabilization time is longer, the next operation must be permitted to follow. This has a large influence particularly on taking a series of continuous shots (continuous shooting). That is, if a time long enough to assure the possible maximum stabilization time is allowed to elapse before the next cycle of shutter opening operation starts, though the camera has no mechanical problems, it becomes impossible to take many shots per unit time. Conversely, if a transition to the shutter opening operation is allowed to take place before the stopping of the film in the preceding cycle of winding operation is stabilized, the image on the film will be blurred, or the shutter mechanism will become unable to be charged, or damaged.